Dark Matter & WIMPs: Why Not Detected in Colliders?

[Vrbic]

If we assume that Dark Matter consists (or their fraction) of new particles, WIMPs. Why they aren't detected in colliders?
I am beginner in a field of DM. Would you recommend some literature?
Thank you all.

 

[ofirg]

First of all, maybe it would be appropriate to move this thread to the High Energy, Nuclear, Particle Physics subforum.

Dark matter signals are looked for in collider experiments, such as ATLAS and CMS.

The role they play in cosmology and astrophysics requires them to be weakly interacting, with no electromagnetic nor strong force/color interactions. Therefore, similar to neutrinos, they are not expected to be detected directly in the detector. However, their presence can be deduced from a momentum imbalance.The topic is quite vast.

Review: http://arxiv.org/pdf/hep-ph/0602187.pdf

Example of a search and the derived limits: http://arxiv.org/pdf/1210.4491.pdf

 

[ChrisVer]

This question is answered simply by experiments. Experiments in colliders have not been able so far to measure "divergencies" from the Standard Model. Also the ways of detection differ by the kind of DM particle you consider.

First of all, the neutrinos indeed are a part of DM, they consist what we call Hot Dark Matter. The "hot" stands from the fact that these particles decoupled while being relativistic (neutrinos decoupled at temperature $T \approx 0.8~MeV$ so since their masses are small compared to this value, they were relativistic) .

Now the data we have, if I recall well my value is from Planck's data, the effective number of neutrino species is $N_{eff}=3.3 \pm 0.27$ (here you can find more on these: http://resonaances.blogspot.de). Comparing to the result one obtains by taking into account finite temperature and decoupling effects and Standard Model predictions, $N_{eff}= 3.046$, you can see that in general they are in par. However this also can allow for extra flavors of more massive particles (in case they are fermions, they are called sterile neutrinos). I don't know exactly how can someone look at them in colliders (maybe by looking for new leptonic processes)...

For example the Supersymmetry gave a candidate for DM, mainly named neutralinos (Lightest supersymmetric particles). For their detection in colliders you get the actual signal and a signal coming from the known Standard Model processes. If you subtract the SM from that signal you can look whether you have an excess of events (coming from new physics) and determine its statistics (whether the excess appears as statistical fluctuations). So far there has not been a detection of such an event excess in the colliders. Mainly we are looking at large missing momenta from the detector, and processes that are not favored in the SM. That is why these particles have not been observed, and the only things you can do is put bounds on the parameter space (the space of the parameters that describe this particle, such as its mass). Reasons for this non-observations vary, they can be much heavier and thus more difficult to create.

The axionic DM candidates are not searched in a collider. There are several experiments, using their features, that have been conducted to look for them. Especially for the DM axions (cosmological axions), their cold-DM-nature depends a lot on the model itself (and what is the relation between inflation reheating and PQ symmetry breaking temperatures or order) and also allows for a certain window of axionic masses (approximately from $10^{-6} \text{to} 10^{-3} ~eV$). For that you need more information about inflation (eg BICEP2 and we are also expecting for Planck's data). In particular depending on the order of the Peccei-Quinn symmetry breaking and inflation, you can have domain walls and strings forming as topological deflects or you may have just one vacuum homogenized by the inflation. Now back to searches of axions, ADMX (microwave cavity experiments) is such an experiment and it exploited the axions' property of coupling with the electromagnetic field. So far they haven't been able to observe an axionic signal, and so they have been only able to put boundaries on its couplings/mass. Better resolution experiments are being prepared for the future searches. According to Peccei the next 10 years there will be more dedicated works in looking for that invisible particle [of course that's a personal opinion].

 

[Chalnoth]

If we assume that Dark Matter consists (or their fraction) of new particles, WIMPs. Why they aren't detected in colliders?
I am beginner in a field of DM. Would you recommend some literature?
Thank you all.

The short answer is that their interactions with normal matter are weak enough that we can't produce them in large enough quantities in colliders to reliably detect the missing mass (the dark matter would look like a chunk of the reaction energy just disappeared, because we wouldn't actually detect the particle itself). As I understand it, while it is conceivable that the LHC could detect dark matter, it really isn't very likely.

 

[websterling]

N_{eff}=3.3 \pm 0.27[/itex] (here you can find more on these: http://resonaances.blogspot.de). Comparing to the result one obtains by taking into account finite temperature and decoupling effects and Standard Model predictions, $N_{eff}= 3.046$, you can see that in general they are in par. However this also can allow for extra flavors of more massive particles (in case they are fermions, they are called sterile neutrinos). I don't know exactly how can someone look at them in colliders (maybe by looking for new leptonic processes).

The latest number combining Planck and Baryon Acoustic Oscillations is Neff =3.04±0.18, in very good agreement with the Standard Model's 3.046, constraining the possibility of additional flavors.

 

[Vrbic]

Thank you all!
I am enriched.

 

[Orodruin]

So far there has not been a detection of such an event excess in the colliders. Mainly we are looking at large missing momenta from the detector, and processes that are not favored in the SM.

Let me just add that probably the most smoking gun here would be monojet events. Theoretically, they could happen, e.g., due to quarks annihilating into a dark matter pair with a gluon emitted by one of the initial state quarks. Lots of missing transverse momenta and just one jet to show for it.

 

[ChrisVer]

The latest number combining Planck and Baryon Acoustic Oscillations is Neff =3.04±0.18, in very good agreement with the Standard Model's 3.046, constraining the possibility of additional flavors.

Well thanks for the updating. In fact I looked to find the exact number online, but the data are many... I used this paper:
http://arxiv.org/pdf/1303.5368v2.pdf
which dates from 2013. In the introduction he gave the number $N_{eff}=3.30 \pm 0.27$ at $68 \% ~CL$ as a result from the combination of WMAP, Planck, Baryon Acoustic Oscillations and high multipole CMB data . In the conclusions, taking into account the BBN theory, they deduce the bound $N_{eff} <3.56$ at $95 \%~CL$ (all these values they give are taken from a 2013 paper as well, cited [31]).

If you have any reference for your number, please let me know. Because , since I'm in uni right now, I also checked this (dates April 2014):
http://iopscience.iop.org/1367-2630/16/6/065002/article
And I could not find any such small $N_{eff}$. Except for a small comment on page 17:

For instance, we know from a joint analysis of BBN and neutrino oscillation data that in order to be compatible with measurements of primordial element abundances, the leptonic asymmetry cannot enhance the neutrino density (at CMB and current time) above [itex]N_{eff} \approx 3.1

and also for Planck+WP+highL+BAO they give again: $N_{eff}= 3.30^{+0.54}_{-0.51}$ at $95 \% ~CL$ (in parallel with higher valued results)

Let me just add that probably the most smoking gun here would be monojet events. Theoretically, they could happen, e.g., due to quarks annihilating into a dark matter pair with a gluon emitted by one of the initial state quarks. Lots of missing transverse momenta and just one jet to show for it.

Well yes. It depends on the channel you are looking into. For example there are people looking into the channels of $\tau \tau \rightarrow \text{hadron+hadron~ or~ hadron+lepton}$ decays. Others were looking (I don't remember if they still do) on Flavor changing neutral currents and compare to SM. Intuitevely what you say also seems plausible.

 

[Vrbic]

My another question was probably said, but generally. Is possible that cold dark matter (I hope I rule out neutrinos) or some of friction is consist of such new particles which can not be created from known particles - SM particles?

 

[websterling]

If you have any reference for your number, please let me know. .

Jester at Resonaances mentioned it-
http://www.resonaances.blogspot.it/2014/12/planck-whats-new.html

I assume that he found it in one of the slide presentations at-
http://www.cosmos.esa.int/web/planck/ferrara2014

There is a graph, but no number, in the release (in French) at-
http://www.insu.cnrs.fr/node/5108

I am downloading the presentations and plan to look through them over the next several days. When I find the one with the number, I'll post a link.

And, if you haven't seen it, the sum of neutrino masses seems to be constrained to <0.23 eV

 

[ChrisVer]

My another question was probably said, but generally. Is possible that cold dark matter (I hope I rule out neutrinos) or some of friction is consist of such new particles which can not be created from known particles - SM particles?

If you consider SM particles the 3 generations of quarks, the 3 generation of leptons, the 4 vector bosons and the Higgs scalar, then CDM does not contain any of these particles...

 

[Orodruin]

I think he means if it is possible that CDM can be created without having a production cross section for any SM particles and I would say yes. In principle there is nothing that demands that it should be possible to create dark matter by colliding SM particles (or at least have a very very very small cross section for such processes). The FIMP (Feebly Interacting Massive Particle) scenario is a scenario where the dark matter has a very small coupling to SM particles. Instead of thermal freeze-out as in the WIMP scenario, dark matter is created through thermal freeze-in in this scenario.

 

[ChrisVer]

Well if that's the case... Then I don't know how you would make such a Lagrangian. For example, why would their couplings to SM particles be so suppressed? Also how were they created? as a distinct sector?
For a practical example, let's say that it is a scalar field, then what doesn't allow it to couple with fermions in a yukawa-interaction? or a pseudoscalar to axial currents?
In any case you would have to start introducing more symmetries. Extra symmetries is a plausible idea, but after a while, this becomes "boring" (without having an actual observed reason to do so, let's say strong CP-problem).. MOND then would be a more plausible idea (even for me ).

 

[Orodruin]

I believe it is more common to assume fermions, in which case you can make a singlet out of a fermion bilinear, but have no SM singlet with dimension one to couple it with. Some theories also include additional gauge groups with neutral gauge bosons that mix with the photon, resulting in very weak couplings.

A relatively nice paper that discusses both the WIMP and FIMP regimes through portal interactions is http://arxiv.org/abs/1112.0493

 

[websterling]

If you have any reference for your number, please let me know.

The number I quoted was from a presentation at the Planck 2014 conference in Ferrara. Neff from various combinations of sources is on slide 13-
http://www.cosmos.esa.int/documents/387566/387653/Ferrara_Dec4_11h50_Lesgourgues_NeutrinosReview.pdf

 

[ChrisVer]

The number I quoted was from a presentation at the Planck 2014 conference in Ferrara. Neff from various combinations of sources is on slide 13-
http://www.cosmos.esa.int/documents/387566/387653/Ferrara_Dec4_11h50_Lesgourgues_NeutrinosReview.pdf

This thing is just crazy All the numbers look pretty low...since it's from Planck 2014, probably I'll have to wait for the Planck's publication in the next days...If that's the case, I'd say that sterile neutrinos seem to get disfavored from the $N_{eff}$ freedom...

 

[websterling]

probably I'll have to wait for the Planck's publication in the next days...I

Probably a bit later than the next days. Planck Collaboration now say not December 22 but by end of January, 2015...
http://www.cosmos.esa.int/web/planck

 

[ChrisVer]

Probably a bit later than the next days. Planck Collaboration now say not December 22 but by end of January, 2015...

And I was expecting it for a presentation in addition to BICEP2...

 

[Orodruin]

To quote Samuel Ting when presenting the AMS-02 results: "Wait!"

 

[websterling]

"Wait!"

The Collaboration now says "in early 2015" I'm still hoping for before the end of the Universe.

And all I wanted for Christmas was the Planck 2014 release and my copy of the new Particle Physics Booklet...

 

[ChrisVer]

I just need to add some correction...

First of all, the neutrinos indeed are a part of DM, they consist what we call Hot Dark Matter.

I said that the neutrinos consist the Hot Dark Matter. This phrase can be easily misunderstood, so I should correct it.
Hot Dark Matter contains neutrinos, however the neutrinos are not the dominant part of it (as a result of the neutrino mass bounds and mass differences).

Eg. from Tritium we know that $m_{\nu_{e}} <2 eV$
Also for $\Omega_\nu = 80 \% \Omega_{mat}$ leads to the sum rule $\sum_{flavors} m_f \approx 12~eV$ and that the neutrino mass differences [from oscillations] are very small [fractions of $eV$ ). So their masses are almost the same, and the sum rule will give around $4eV$ per flavor... this violates the contraint for the electron neutrino obtained from tritium decays...
So neutrinos can be part of the HDM but not the dominant piece of it.

 

[vanhees71]

BTW: Has anybody got the copy of the large book (Review of Particle Physics) yet? On their website the PDG says they were sent out and the booklets are in print.

 

[websterling]

BTW: Has anybody got the copy of the large book (Review of Particle Physics) yet? On their website the PDG says they were sent out and the booklets are in print.

Mine arrived December 5; the PDG site said that it was mailed on October 3. Says the booklet was sent December 8- haven't received it yet.

The book/booklet are being produced in China- mine actually came through an importer in California. The importing process is probably causing delays in delivery, maybe more so in some countries.